For a project we are developing at PoliMI/USI, we are using Yahoo! APIs to get data (photos and tags associated to these photos) about a city. We thought it would be nice to provide, together with this information, also a link or an excerpt from the Wikipedia page that matches the specific city. However, we found that the matching between Yahoo's WOEIDs and Wikipedia articles is far from trivial...

First of all, just two words on WOEIDs: they are unique, 32-bit identifiers used within Yahoo! GeoPlanet to refer to all geo-permanent named places on Earth. WOEIDs can be used to refer to differently sized places, from towns to Countries or even continents (i.e. Europe is 24865675). A more in-depth explanation of this can be found in the Key Concepts page within GeoPlanet documentation, and an interesting introductory blog post with examples to play with is available here. Note that, however, you now need a valid Yahoo! application id to test these APIs (which means you should be registered in the Yahoo! developer network and then get a new appid by creating a new project).

One cool aspect of WOEIDs (as for other geographical ids such as GeoNames' ones) is that you can use them to disambiguate the name of a city you are referring to: for instance, you have Milan and you want to make sure you are referring to Milano, Italy and not to the city of Milan, Michigan. The two cities have two different WOEIDs, so when you are using one of them you exactly know which one of the two you are talking about. A similar thing happens when you search for Milan (or any other ambiguous city name) on Wikipedia: most of the times you will be automatically redirected to the most popular article, but you can always search for its disambiguation page (here is the example for Milan) and choose between the different articles that are listed inside it.

Of course, the whole idea of having standard, global, unique identifiers for things in the real world is a great one per se, and being able to use it for disambiguation is only one aspect of it. While disambiguation can be (often, but not always!) easy at the human level, where the context and the background of the people who communicate help them in understanding which entity a particular name refers to, this does not hold for machines. Having unique identifiers saves machines from the need of disambiguating, but also allows them to easily link data between different sources, provided they all use the same standard for identification. And linking data, that is making connections between things that were not connected before, is a first form of inference, a very simple but also a very useful one that allows us to get new knowledge from the one we originally had. Thus, what makes these unique identifiers really useful is not only the fact that they are unique. Uniqueness allows for disambiguating, but is not sufficient to link a data source to others. To do this, identifiers also need to be shared between different systems and knowledge repositories: the more the same id is used across knowledge bases, the easier it is to make connections between them.

What happens when two systems, instead, use different ids? Well, unless somebody decides to map the ids between the two systems, there are few possibilities of getting something useful out of them. This is the reason why the reconciliation of objects across different systems is so useful: once you state that their two ids are equivalent, then you can perform all the connections that you would do if the objects were using the same id. This is the main reason why matching WOEIDs for cities with their Wikipedia pages would be nice, as I wrote at the beginning of this post.

Wikipedia articles are already disambiguated (except, of course, for disambiguation pages) and their names can be used as unique identifiers. For instance, DBPedia uses article names as a part of its URIs (see, for instance, information about the entities Milan and Milan(disambiguation)). However, what we found is that there is no trivial way to match Wikipedia articles with WOEIDs: despite what others say on the Web, we found no 100% working solution. Actually, the ones who at least return something are pretty far from that 100% too: Wikilocation works fine with monuments or geographical elements but not with large cities, while Yahoo! APIs themselves have a direct concordance with Wikipedia pages, but according to the documentation this is limited to airports and towns within the US.

The solution to this problem is a mashup approach, feeding the information returned by a Yahoo! WOEID-based query to another data source capable of dealing with Wikipedia pages. The first experiment I tried was to query DBPedia, searching for articles matching Places with the same name and a geolocation contained in the boundingBox. The script I built is available here (remember: to make it work, you need to change it entering a valid Yahoo! appid) and performs the following SPARQL query on DBPedia:

Basically, what it gets are the Wikipedia page and the Freebase URI for a place called "like" the one we are searching, where "like" means either exactly the same name ("Milan") or one which still begins with the specified name but is followed by a comma and some additional text (i.e. "Milan, Italy"). This is to take into account cities whose Wikipedia page name also contains the Country they belong to. Some more notes are required to better understand how this works:

I am querying for articles matching "Places" and not "Cities" because on DBPedia not all the cities are categorized as such (data is still not very consistent);

I am matching rdfs:label for the name of the City, but unfortunately not all cities have such a property;

requiring the Wikipedia article to have equivalent URIs related with the owl:sameAs property is kind of strict, but I saw that most of the cities had not only one such URI, but also most of the times the one from Freebase I was searching for.

This solution, of course, is still kind of naive. I have tested it with a list WOEIDs of the top 233 cities around the world and its recall is pretty bad: out of 233 cities the empty results were 96, which corresponds to a recall lower than 60%. The reasons of this are many: sometimes the geographic coordinates of the cities in Wikipedia are just out of the bounding box provided by GeoPlanet; other times the city name returned by Yahoo! does not belong to any of the labels provided by DBPedia, or no rdfs:label property is present at all; some cities are not even categorized as Places; very often accents or alternative spellings make the city name (which usually is returned by Yahoo! without special characters) untraceable within DBPedia; and so on.

Trying to find an alternative approach, I reverted to good old Freebase. Its api/service/search API allows to query the full text index of Metaweb's content base for a city name or part of it, returning all the topics whose name or alias match it and ranking them according to different parameters, including their popularity in Freebase and Wikipedia. This is a really powerful and versatile tool and I suggest everyone who is interested in it to check its online documentation to get an idea about its potential. The script I built is very similar to the previous one: the only difference is that, after the query to Yahoo! APIs, it queries Freebase instead of DBPedia. The request it sends to the search API is like the following one:

where (like in the previous script) city name and bounding box coordinates are provided by Yahoo! APIs. Here are some notes to better understand the API call:

the city name is provided as the query parameter, while type is set to /location/citytown to get only the cities from Freebase. In this case, I found that every city I was querying for was correctly assigned this type;

the mql_output parameter specifies what you want in Freebase's response. In my case, I just asked for Wikipedia ID (asking for the "key" whose "namespace" was /wikipedia/en_id). Speaking about IDs, Metaweb has done a great job in reconciliating entities from different sources and already provides plenty of unique identifiers for its topics. For instance, we could get not only Wikipedia and Freebase own IDs here, but also the ones from Geonames if we wanted to (this is left to the reader as an exercise ;)). If you want to know more about this, just check the Id documentation page on Freebase wiki;

the mql_filter parameter allows you to specify some constraints to filter data before they are returned by the system. This is very useful for us, as we can put our constraints on geographic coordinates here. I also specified the type /location/location to "cast" results on it, as it is the one which has the geolocation property. Finally, I repeated the constraint on the Wikipedia key which is also present in the output, as not all the topics have this kind of key and the API wants us to filter them away in advance.

Luckily, in this case the results were much more satisfying: only 9 out of 233 cities were not found, giving us a recall higher than 96%. The reasons why those cities were missing follow:

three cities did not have the specified name as one of their alternative spellings;

four cities had non-matching coordinates (this can be due either to Metaweb's data or to Yahoo's bounding boxes, however after a quick check it seems that Metaweb's are fine);

two cities (Buzios and Singapore) just did not exist as cities in Freebase.

The good news is that, apart from the last case, the other ones can be easily fixed just by updating Freebase topics: for instance one city (Benidorm) just did not have any geographic coordinates, so (bow to the mighty power of the crowd, and of Freebase that supports it!) I just added them taking the values from Wikipedia and now the tool works fine with it. Of course, I would not suggest anybody to run my 74-lines script now to reconciliate the WOEIDs of all the cities in the World and then manually fix the empty results, however this gives us hope on the fact that, with some more programming effort, this reconciliation could be possible without too much human involvement.

So, I'm pretty satisfied right now. At least for our small project (which will probably become the subject of one blog post sooner or later ;)) we got what we needed, and then who knows, maybe with the help of someone we could make the script better and start adding WOEIDs to cities in Freebase... what do you think about this?

I have prepared a zip file with all the material I talked about in this post, so you don't have to follow too many links to get all you need. In the zip you will find:

woe2wp.pl and woe2wpFB.pl, the two perl scripts;

test*.pl, the two test scripts that run woe2wp or woe2wpFB over the list of WOEIDs provided in the following file;